Oxygen is separated from oxygen containing feeds, such as air, through cryogenic rectification. In order to operate a cryogenic rectification plant, refrigeration must be supplied to offset ambient heat leakage, warm end heat exchange losses and to allow the extraction or production of liquid products, including liquid oxygen, liquid nitrogen, or liquid argon. While the main source of refrigeration for a cryogenic rectification plant is typically supplied by expanding part of the feed air stream or a waste stream to generate a cold stream that is then introduced into the main heat exchanger or the distillation column, external refrigeration can also be imparted by other refrigerant streams introduced into the main heat exchanger, including a refrigerant stream from a closed loop supplemental refrigeration cycles as generally described in U.S. Pat. No. 8,397,535.
One of the limitations or drawbacks of the existing supplemental refrigeration cycles used in air separation plants is that the centrifugal compressors and turboexpanders in such supplemental refrigeration circuits are generally operating in an ‘on’ or ‘off’ mode. In other words, the centrifugal compressors and turboexpanders are either operating so as produce the supplemental refrigeration and additional liquid product make or are shut down thereby not producing supplemental refrigeration and foregoing any additional liquid product make. The continued cycling of the centrifugal compressors and turboexpanders between operating mode and shut-down mode adversely impacts the overall efficiency and reliability of the supplemental refrigeration cycle.
A small degree of adjustment in existing supplemental refrigeration circuits may be achieved through the adjustment of compressor inlet guide vanes. However, one must be careful of adjustments that would sent the compressor into a surge condition or a stonewall conditions as a result of too little or too much flow to the compressor. As a result, the existing or prior art supplemental refrigeration circuits are generally operated at a fixed or near-fixed operating point. This inability to modulate the level of supplemental refrigeration over broad operating ranges effectively limits the plant operator from precisely controlling the amount of liquid product produced by the air separation plant at any given time.
What is needed, therefore, is a supplemental refrigeration circuit or system adapted for use in air separation plants that facilitates modulating the level of supplemental refrigeration produced over broad operating ranges and thus allows more precise control of the amount of liquid product produced by the air separation plant at any given moment.